1. Field of the Invention
The present invention relates to a process for producing a modified acrylic rubber and more particularity to a process for producing a peroxide-crosslinkable, modified acrylic rubber.
2. Description of Related Art
Acrylic rubbers are widely used as molding materials for hoses, various seal materials, etc. due to its relative low production cost as well as good heat resistance and oil resistance, but requirements for its higher functionability and lower production cost are still now in increasing demand.
For lower production cost, it is highly desired to shorten the processing time and molding time of crosslinkable compositions, and various attempts have been so far made for that purpose. Attempts to increase the crosslinking speed, which relates to the shortening of molding time, inevidently deteriorates the scorch resistance and thus a more practical, stable crosslinking systems are in keen demand.
Systems capable of satisfying both high speed crosslinkability and scorch resistance at the same time include a peroxide-based crosslinking system, which has been so far widely studied but still has problems.
(1) A process for copolymerizing dienes, for example, 5-alkylidene-2-norbornene, etc. to introduce crosslinkable unsaturated groups into side chains (JP-A-49-87787) has such a disadvantage as poor processability, etc. of the product due to interreaction of the crosslinkable unsaturated groups during the copolymerization reaction to cause intermolecular bridging.
(2) A process for crosslinking reaction between the side chains of alkoxyalkyl acrylate in the acrylic copolymers and a bismaleimide compound used as a crosslinking aid (JP-A-5-214196) has such a disadvantage as poor compression set when subjected to vulcanization molding into O-rings.
(3) Acrylic rubber containing iodine or bromine as crosslinking sites can be crosslinked with a peroxide, but the iodine- or bromine-containing, crosslinkable monomers to be introduced into copolymers are generally expensive with failure to satisfy the requirements for lower production cost.
(4) A process for graft polymerization of monomers having a polymerizable, unsaturated group can be used as a procedure for introducing crosslinkable unsaturated groups (JP-A-3-221513), but use of an organic solvent in the reaction is with failure to satisfy the requirements for lower cost.
An object of the present invention is to provide a process for producing a modified acrylic rubber in the absence of a solvent, the modified acrylic rubber so produced being distinguished in the molding charactertics and O-ring compression set.
The object of the present invention can be attained by a process for producing a modified acrylic rubber, which comprises mixing an acrylic rubber having reactive functional groups with an unsaturated compound reactive with the reactive functional group with heating in the absence of a solvent.
An acrylic rubber having reactive groups for use in the present invention includes, for example, acrylic copolymers obtained by copolymerizing at least one of alkyl acrylate and alkoxyalkyl acrylate as the main component with about 0.5 to about 10% by weight, preferably about 1% to about 8% by weight, on the basis of the acrylic copolymer, of a monomer having an active chlorine group, a carboxyl group, an epoxy group, an active ester group or the like.
Alkyl acrylate for use in the present invention includes, for example, alkyl acrylates having an alkyl group with 1 to 8 carbon atoms (the alkyl group may further having a substituent such as a cyano group, etc.) such as methyl acrylate, ethyl acrylate, n- or iso-propyl acrylate, n- or iso-butyl acrylate, n-amyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, 2-cyanoethyl acrylate, etc., preferably ethyl acrylate and n-butyl acrylate, and further methyl methacrylate, ethyl methacrylate and n-butyl methacrylate.
Alkoxyalkyl acrylate for use in the present invention includes, for example, alkoxyalkyl (meth)acrylates having an alkoxyalkyl group with 2 to 8 carbon atoms such as methoxymethyl acrylate, ethoxymethyl acrylate, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-butoxyethyl acrylate, and the corresponding mathacrylates, etc., preferably 2-methoxyethyl acrylate and 2-ethoxyethyl acrylate.
At least one of these alkyl acrylates and alkoxyalkyl acrylates can be used in a proportion of 99 to 50% by weight, preferably 98 to 70% by weight, as the main component of the acrylic copolymer, and both alkyl acrylate and alkoxyalkyl acrylate, when used together, can be generally in a ratio of the former to the latter of about 90 to about 10: about 10 to about 90% by mole.
A portions particularly up to about 20% by weight of the acrylic copolymer can be replaced with other copolymerizable monomer, followed by copolymerization of the resulting mixture. Such other copolymerizable monomer includes, for example, ethylene, propylene, vinyl chloride, vinylidene chloride, acrylonitrile, styrene, vinyl acetate, ethyl vinyl ether, butyl vinyl ether, alkyl methacrylate, alkoxyalkyl methacrylate, etc.
Monomers (vinyl monomers) having a reactive functional group, which are to be copolymerized with the main component into the acrylic rubber include, for example, monomers having an active chlorine group, a carboxyl group, an epoxy group, an active ester group, etc.
The monomers having an active chlorine group include, for example, 2-chloroethyl vinyl ether, 2-chloroethyl acrylate, vinyl monochloroacetate, chloromethylstyrene, etc. The monomers having a carboxyl group include, for example, (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, maleic acid monoester, fumaric acid monoester, etc. The monomers having an epoxy group include, for example, allyl glycidyl ether, glycidyl vinyl ether, glycidyl (meth)acrylate, (meth)acrylates containing an alicyclic epoxy group, etc. The monomers having an active ester group include, for example, (meth)acrylates having a phenyl group such as phenyl (meth)acrylate, cyanophenyl (meth)acrylate, nitrophenyl (meth)acrylate, etc.
Unsaturated compounds reactive with the reactive functional group originating from these monomers copolymerized into the acrylic copolymer include the following compounds.
Unsaturated compounds reactive with the active chlorine group include, besides the above-mentioned monomers having a carboxyl group, such unsaturated compounds having a carboxyl group as:
CH2xe2x95x90CRCOOR1(OCOR2)nCOOH,
CH2xe2x95x90CRCOOR1OCOPhCOOH,
where R represents a hydrogen atom or a methyl group; R1 and R2 each represent an alkylene group having 1 to 6 carbon atoms; Ph represents a phenylene group; and n is an integer of 1 to 6,
vinyl acetic acid,
2-(meth)acryloyloxyethyl succinate,
2-(meth)acryloyloxyoxyethyl phthalate,
2-(meth)acryloyloxyethyl hexahydrophthalate; and, 
(PAB-MI, trademark of a product from Mitsui Chemical Corporation, Japan).
Reaction of the unsaturated compound with the acrylic copolymer having active chlorine groups can be carried out in the presence of such an acid acceptor as calcium hydroxide, magnesium hydroxide, synthetic hydrotalcite, sodium stearate, potassium stearate, sodium 2-ethylhexanoate, etc. and typically such a quaternary onium salt as benzyltriphenylphosphonium chloride, tetrabutylammonium bromide, etc. Furthermore, unsaturated compounds containing a thiol group such as allylmercaptan, etc. can be used and also subjected to the reaction in the presence of an acid acceptor and a quaternary onium salt.
Unsaturated compounds reactive with the carboxyl group of the acrylic copolymer having carboxyl groups include, for example, unsaturated compounds having a primary amino group such as allylamine, aminostyrene, aminomethylstyrene, p-aminophenylene maleimide, etc. Reaction of the unsaturated compound with the acrylic copolymer having a carboxyl group can be carried out in the presence of a strongly basic compound such as guanidine, etc.
Unsaturated compounds reactive with the epoxy group of the acrylic copolymer include, for example, the above-mentioned monomers having an active ester group. Reaction of the unsaturated compound with the epoxy group of the acrylic copolymer having epoxy groups can be carried out in the presence of a quaternary onium salt. Furthermore, the above-mentioned unsaturated compounds having a primary amino group or a carboxyl group can be also used.
Unsaturated compounds reactive with the active ester group of acrylic copolymer having active ester groups include, for example, the above-mentioned monomers having an epoxy group. Reaction of the unsaturated compound with the active ester group of the acrylic copolymer having active ester groups can be carried out in the presence of a quaternary onium salt.
At least one of ammonium salts and phosphonium salts, represented by the following formulae, respectively, can be used as a quaternary onium salt serving as a catalyst for most of the above-mentioned reactions:
(R1R2R3R4N)+Xxe2x88x92
and
xe2x80x83(R1R2R3R4P)+Xxe2x88x92
wherein R1 to R4 each represent an alkyl group, an alkoxy group, an aryl group, an alkylaryl group, an aralkyl group or a polyoxyalkylene group, having 1 to 25 carbon atoms, respectively, 2 or 3 of whose groups may form a heterocyclic structure together with P or N; Xxe2x88x92 represents an anion such as Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, HSO4xe2x88x92, H2PO4xe2x88x92, RCOOxe2x88x92, ROSO2xe2x88x92, RSOxe2x88x92, ROPO2Hxe2x88x92, CO3xe2x88x92xe2x88x92, etc.
Specifically, they include such quaternary ammonium salts as tetraethylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium iodide, n-dodecyltrimethylammonium bromide, cetyldimethylbenzylammonium chloride, methylcetyldibenzylammonium bromide, cetyldimethyl-ethylammonium bromide, octadecyltrimethylammonium bromide; cetylpyridinium chloride, cetylpyridinium bromide, cetylpyridinium iodide, cetylpyridinium sulfate, 1-benzylpyridinium chloride, 1-benzyl-3,5-dimethylpyridinium chloride, 1-benzyl-4-phenylpyridinium chloride, 1,4-dibenzylpyridinium chloride, 1-benzyl-4-(pyrrolidinyl)pyridinium chloride, 1-benzyl-4-pyridinopyridinium chloride; tetraethylammonium acetate, trimethylbenzylammonium benzoate, trimethylbenzylammonium p-toluenesulfonate, trimethylbenzylammonium borate, 8-benzyl-1,8-diazabicyclo[5.4.0]-undec-7-enium chloride, 1,8-diazabicyclo[5.4.0]-undecene-7-methylammonium methosulfate, 5-benzyl-1,5-diazabicyclo[4.3.0]-5-nonenium chloride, 5-benzyl-1,5-diaza-bicyclo[4.3.0]-5-nonenium bromide, 5-benzyl-1,5-diazabicyclo[4.3.0]-5-nonenium tetrafluoroborate, 5-benzyl-1,5-diazabicyclo[4.3.0]-5-nonenium hexafluorophosphate, etc.; and such quaternary phosphonium salts as tetraphenylphosphonium chloride, triphenylbenzylphosphonium chloride, triphenylbenzylphosphonium bromide, triphenylmethoxymethylphosphonium chloride, triphenylmethyl-carbonylmethylphosphonium chloride, triphenylethoxycarbonyl-methylphosphonium chloride, trioctylbenzylphosphonium chloride, trioctylmethylphosphonium bromide, trioctylethylphosphonium acetate, trioctylethylphosphonium dimethylphosphate, tetraoctyl-phosphonium chloride, cetyldimethylbenzylphosphonium chloride, etc.
These quaternary onium salts or guanidine or the like can be used in a proportion of about 0.05 to about 20 parts by weight, preferably about 0.1 to about 10 parts by weight, per 100 parts by weight of the acrylic rubber.
Reaction of the acrylic rubber having reactive functional groups with the unsaturated compound reactive with the reactive functional group in the presence of such a catalyst can be carried out by mixing these two reactants with heating in absence of a solvent.
The mixing with heating can be carried out by kneading at a temperature of about 50xc2x0 to about 200xc2x0 C., preferably about 80xc2x0 to about 150xc2x0 C. for about 1 to about 10 minutes with an open roll, an enclosed kneader, an extruder, etc., because of the reaction in the absence of a solvent. Actually, the acrylic rubber having reactive functional groups is charged into or wound up at a kneader preheated to such a temperature and subjected to conduct mastication for about one minute, and then the unsaturated compound and the catalyst are added thereto and subjected to intimate kneading, where a polymerization inhibitor such as hydroquinone, methoquinone, phenothiazine, etc., or an antioxidant such as hindered phenol, tertiary amine, etc can be appropriately added thereto, if required, to protect the unsaturated groups.
The resulting modified acrylic rubber is made susceptible to peroxide crosslinking by adding about 0.1 to about 10 parts by weight, preferably about 1 to about 5 parts by weight of an organic peroxide as a crosslinking agent per 100 parts by weight of the modified acrylic rubber to form a crosslinkable composition. The crosslinkable composition can appropriately further contain a filler or a reinforcing agent such as carbon black, silica, etc., a crosslinking aid such as triallyl isocyanurate, trimethylolpropane trimethacrylate, etc. and other necessary additives. The modified acrylic rubber can be crosslinked with other crosslinking agents than the organic peroxide, such as sulfur, polyol, etc.
The crosslinkable composition can be prepared by kneading the modified acrylic rubber, the crosslinking agent, the filler, etc. with an open roll, an enclosed kneader, etc. and can be press vulcanized at a temperature of about 150xc2x0 to about 200xc2x0 C. for about 1 to about 30 minutes or, if necessary, can be oven vulcanized at a temperature of about 150xc2x0 to about 200xc2x0 C.
The present modified acrylic rubber is distinguished in not only molding characteristics, but also normal state physical properties, particularly strength at break and elongation at break of the vulcanization product, as compared with graft copolymers using an organic solvent. Furthermore, the present modified acrylic rubber is also distinguished in production cost because no organic solvent is used.